Answer to Question #8995 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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Q

Does MgCl2 absorb or thermalize neutron radiation? I work for a state department of transportation and have been wondering whether the MgCl2 that we use for dust control on our construction projects affects the moisture content recorded by the Troxler nuclear gauges used for density control. The Troxler gauge uses a 1.48 × 109 Bq Am-Be source as a neutron emitter and 3He detector tube for detection of thermalized neutrons. The 3He tube is insensitive to fast neutrons and the counts obtained are directly proportional to the hydrogen (water) content of the material and displayed as percent moisture content. Hydrogen has a cross-sectional area of 0.33, whereas chlorine has an area of 35.3. Will this result in higher or lower moisture readings?

A

Your interesting question has two major points for consideration. The first relates to possible enhancement of thermal neutron production leading to overestimation of water content, and the second involves the reduction of thermal neutrons by absorption in chlorine leading to an underestimation of water content. I shall attempt to address each of these.

The neutrons produced by the Am-Be source are relatively high in energy, having an average energy of about 4.5 MeV. As you know, the Troxler system works by measuring neutrons that have been greatly reduced in energy, some being thermalized, primarily by elastic scattering interactions with hydrogen, obviously a major atomic constituent of water. While elastic scattering (as well as some inelastic scattering) events may occur with the nuclei of other atomic species that are present, including chlorine in MgCl2, such interactions are not nearly as effective as scatter with hydrogen in reducing energies to thermal energies. This is because the energy transfer in elastic scattering is most efficient when the scatterer has a mass that is very similar to that of the neutron; hydrogen fills the bill here. For example, in a head-on collision with a hydrogen nucleus, a neutron may lose all of its kinetic energy; in a similar collision with chlorine, the neutron could lose no more than about 11% of its energy. It would take approximately 150 sequential elastic scattering events with chlorine to reduce the average energy neutron from Am-Be to thermal energies, while approximately 25 such collisions with hydrogen would accomplish the same energy reduction. I believe it is fair to judge that the presence of MgCl2 will not be a significant factor in enhancing the response of the detector.

You are correct in noting that the cross section for thermal neutron absorption is about 100 times greater for chlorine than for hydrogen. As to whether chlorine might interfere with measurements by way of capturing thermal neutrons before they can interact in the detector and thus depress the response, possibly leading to an underestimate of water content, this will depend on the amount of MgCl2 dispersed between the ground and the detector.

I don't know much about the use of MgCl2 for dust control, nor do I know how much you are using for that purpose but, based on material I could access on the internet, it appears that a usage of approximately one pound per square yard of surface area is a usual amount for many dust-control purposes, although rates may be up to twice this amount. The one pound per square yard concentration corresponds to a dispersal rate of 54 mg cm-2 (0.054 g cm-2).

If we assume, for estimation, that all of the thermal neutrons moving toward the detector must traverse this thickness, we can estimate the thermal neutron transmission fraction as e-SmT, where Sm is the mass macroscopic thermal neutron absorption cross section, in cm2 g-1, and T represents the mass density thickness of the layer, 0.054 g cm-2. The atomic thermal neutron cross section for Cl is 35.3 barns and that for Mg is 0.06 barns, from which we obtain 70.7 barns, or 7.07 x 10-23 cm2, as the thermal neutron absorption cross section per molecule of MgCl2. The molecular weight of MgCl2 is 95.21. Since this mass in grams contains Avogadro's number of molecules, it is a simple step to calculate the mass macroscopic cross section of MgCl2 as (7.07 x 10-23 cm2/molecule)(6.022 x 1023 molecules/95.21 grams) = 0.447 cm2 g-1.

We now estimate the thermal neutron transmission fraction as
e-(0.447 cm2 g-1)(0.054 g cm-2) = 0.976. Thus, for this simple case of what I believe is a reasonable thickness of MgCl2, we expect a reduction in thermal neutrons of about 2.4%. At the two pounds per square yard level the transmission fraction would be 0.953, implying a thermal neutron detection loss of 4.7%.

In conclusion, the impact of the MgCl2 does not appear large, but it may not be inconsequential, depending on the amount of MgCl2 being used and the degree of uncertainty you are willing to accept in your determinations. I hope this helps.

George Chabot, PhD, CHP

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